Surface treatment of magnesium and its alloys

ABSTRACT

Methods of polishing and/or brightening surfaces of magnesium or magnesium alloy are disclosed. Polishing and/or brightening methods suitable to both high and low aluminium content magnesium articles are disclosed. In each of the methods, the surface is polished and then passivated. Using the disclosed methods it is possible to control aspects of the appearance of the surface to thereby, for example, obtain a bright and shiny surface to a magnesium or magnesium alloy article which is both stable and corrosion resistant. It is also possible to provide a predetermined texture to the surface using optional steps.

TECHNICAL FIELD

The present invention relates to surface treatment of magnesium and itsalloys and in particular, though not solely, this invention relates toproviding a bright shiny and/or textured surface finish to magnesium ormagnesium alloy.

BACKGROUND ART

Magnesium metal, either pure or in an alloy form, is a highly reactivemetal. In its usual fabricated forms the surface appearance isrelatively dull and obscured by surface oxidation products or othercompounds. Accordingly, magnesium metal does not usually manifest abright, shiny, metallic finish except when the article has beenmachined. Although a dull surface may not always be disadvantageous, itis often desirable to provide a bright or polished surface to magnesiumfor reasons of aesthetics or utility. Owing to the reactivity ofmagnesium, post-treatments are generally necessary to passivate thesurface, and conventionally the desired surface finish is lost duringthis step.

Conventional treatment of magnesium and its alloys has mostly consistedof treatments that result in an anodic coating being applied to themetal, or a paint film, following some kind of pre-treatment processintended to promote paint adhesion. Mostly these processes neitherbrighten nor polish the metal. Anodising processes for magnesium differfrom those employed for aluminium in that the resulting surface isopaque and frequently the coating is inherently coloured. While in manyapplications this may be a desirable finish in itself, this is notregarded as surface polishing or brightening.

Techniques for applying paint to magnesium or its alloys comprise in themain pre-treatments such as chromating or non-chromating conversiontreatments, followed by application of a paint. The paint may be in theform of a powder coat, which is electrostatically applied thenoven-cured, or a wet paint. The painted surface may have a shiny finish,but it is not a finish that is comparable to a shiny metallic surface.

Techniques for chemically or electrochemically brightening other metalsexist. Methods for brightening aluminium alloy articles are commonlyemployed in anodising plants. However, these techniques are notapplicable to magnesium alloys due to their different chemistry. Forexample, an approach to brightening aluminium involves a combined acidmixture in which magnesium articles would dissolve rapidly.

Many different metals and alloys of aluminium can be brightened byeither chemical or electrochemical methods. Although, many brighteningmethods have been known for a long time, little work has been made toaccommodate magnesium and its alloys.

Two other problems exist that make the processing of magnesium andmagnesium alloys difficult. Firstly, many commercial alloy articles(especially die cast articles) exhibit significant amounts ofsegregation both of the surface and in the interior of the article. Thistogether with commonly encountered casting defects produces a highlyinhomogeneous surface which cannot be improved by conventional means.Secondly, as magnesium is a highly reactive metal that forms anon-continuous oxide layer, for a viable bright surface finish to beattained the surface must be treated in a manner to prevent the oxidelayer forming after surface brightening or polishing treatments.Traditionally such treatments include films or coatings which are opaqueand/or are produced using heavy metals and other undesirable toxicchemicals.

It is also possible in some circumstances to chemically orelectrochemically plate other metals, for instance, nickel, ontomagnesium substrates. Although the resulting plated article has a shiny,metallic appearance, this is due to the plated metal and could not beregarded as a system for brightening or polishing magnesium or magnesiumalloy articles. There are also profound disadvantages in plating othermetals onto magnesium or magnesium alloy substrates including forexample galvanic corrosion problems in corrosive environments, expense,processing difficulties in a plating process and high reject rates frommost processes.

It is therefore an object of the present invention to provide a surfacetreatment for magnesium and its alloys which will go at least some waytowards addressing the foregoing problems or at least to provide theindustry and/or public with a useful choice.

All references, including any patents or patent applications cited inthis specification are hereby incorporated by reference. No admission ismade that any reference constitutes prior art. The discussion of thereferences states what their authors assert, and the applicants reservethe right to challenge the accuracy and pertinence of the citeddocuments. It will be clearly understood that, although a number ofprior art publications are referred to herein, this reference does notconstitute an admission that any of these documents form part of thecommon general knowledge in the art, in New Zealand or in any othercountry.

It is acknowledged that the term ‘comprise’ may, under varyingjurisdictions, be attributed with either an exclusive or an inclusivemeaning. For the purpose of this specification, and unless otherwisenoted, the term ‘comprise’ shall have an inclusive meaning—i.e. that itwill be taken to mean an inclusion of not only the listed components itdirectly references, but also other non-specified components orelements. This rationale will also be used when the term ‘comprised’ or‘comprising’ is used in relation to one or more steps in a method orprocess.

Further aspects and advantages of the present invention will becomeapparent from the ensuing description which is given by way of exampleonly.

DISCLOSURE OF INVENTION

Accordingly, in a first aspect the invention consists in a method ofpolishing and/or brightening a magnesium or magnesium alloy surfacecomprising the steps of:

-   -   i) polishing the surface, and    -   ii) passivating the polished surface.

Preferably, the method comprises an initial step of pre-treating of saidsurface to remove surface contaminants.

Preferably, said pre-treatment step comprises chemically etching saidsurface and/or degreasing said surface.

Preferably, surface contaminants are removed prior during thepre-treatment step by contacting said surface with one or moredegreasing components, such as sodium hydroxide.

Preferably, said chemical etching component comprises a nitric acidand/or phosphoric acid.

Preferably, said pre-treatment step includes chemically blasting saidsurface.

Preferably, said polishing step is carried out by a chemical polishand/or electrochemical polish while said surface is immersed in apolishing composition.

Preferably, said chemical polish and/or electrochemical polish removessurface layers and/or reduces microscopic high points from the surface.

Preferably, said polishing step is carried out by immersing said surfacein a bath comprising one or more of the following components; aphosphoric acid solution, monopropylene glycol, ethylene glycol, andnitric acid.

Preferably, said electrochemical polish is a galvanic electrolysis.

Preferably, said electrochemical polish further includes the supply ofan external voltage to said surface.

Preferably, during said electrochemical polish step an electrolyteanti-stagnation means is utilised or an AC voltage is applied to theelectrolyte containing said surface.

Preferably, said electrolyte anti-stagnation means is an electrolytestirrer and/or an ultrasonic wave generating means.

Preferably, said polishing step is followed by an intermediary washremoving at least some of the chemical and/or electrolyte solution fromsaid surface.

Preferably, said intermediary wash is carried out in a compositioncontaining monopropylene glycol and/or ethylene glycol.

Preferably, said polishing step and/or said intermediary wash isfollowed by an alkaline wash.

Preferably, said alkaline wash is carried out in a compositioncontaining sodium hydroxide.

Preferably, said alkaline wash substantially neutralises acids and/orsubstantially removes Aluminium, Manganese or Zinc from said surface.

Preferably, said passivating step provides a substantially corrosionresistant and/or water insoluble surface coating or film.

Preferably, said substantially corrosion resistant and/or waterinsoluble surface coating or film is a phosphate salt coating or film.

Preferably, an inorganic material coating or sealer is applied to saidsubstantially corrosion resistant and/or water insoluble surface coatingor film.

Preferably, said inorganic material coating or sealer is substantiallytransparent and/or substantially provides corrosion protection and/or atleast some protection from mechanically induced damage.

Preferably, said inorganic material coating or sealer is a silicon basedcomposition, such as a disodium metasilicate, and a polyacrylamidecoagulant in de-ionised water.

Preferably, said passivating step and/or said inorganic material coatingor sealer step is followed by a surface drying step.

Preferably, wherein a chemical blasting step comprises the steps of:

-   -   a. immersing the surface in an iron based solution,    -   b. activating said surface with said iron based solution,        wherein said iron based solution is reduced to thereby deposit        iron on said surface,    -   c. etching said surface with an etch composition to modify the        activated surface layer,    -   d. stripping iron deposits from said surface with an iron        removal composition, and    -   e. washing said surface to substantially remove compositions        remaining on said surface.

Preferably, said activator is a solution selected from the following;ferric chloride, hydrochloric acid, ammonium bifluoride, and ammoniumbromide.

Preferably, said etch composition is selected from the following; ferricchloride; ferric chloride and phosphoric acid solution, or a reducedsolution of ferric chloride and phosphoric acid.

Preferably, said iron removal composition is selected from thefollowing; nitric acid and sodium borate in solution, or nitric acid andphosphoric acid in solution.

Preferably, said step of washing said surface is carried out with awater wash or an alkaline wash.

BRIEF DESCRIPTION OF DRAWINGS

Further aspects of the present invention will become apparent from thefollowing description which is given by way of example only and withreference to the accompanying drawings in which:

FIG. 1 illustrates a process flow diagram of various process steps ofone embodiment according to the present invention, and

FIG. 2 illustrates a passivation cell configuration of an embodimentaccording to the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

A set of method steps for chemically and/or electrochemicallybrightening or texturing (also termed “polishing” herein) the surface ofarticles composed of magnesium or its alloys are described herein. Thesteps making up the processes by which the articles are polished as wellas compositions of the chemical solutions that are used from the presentinvention. The following description is accordingly given by way ofexample only and it should be appreciated that a number of functionalequivalents can be substituted for the compositions.

With reference to the drawings and in particular FIG. 1, a flowchart ofvarious steps involved in the polishing and/or brightening of amagnesium or magnesium alloy surface is shown.

Prior to processing, it is usually appropriate to pre-treat (steps 1 to4) magnesium or magnesium alloy articles to clean and prepare thesurfaces for brightening or polishing. The treatment depends on alloy,and the most preferred treatments are summarised below for some commonalloys and forming processing. Alternative treatments may be possibleand in some cases, depending on the original surface condition of thearticle, the omission of one or more steps may be possible withoutadverse effect on subsequent processing steps.

This invention discloses a means for processing and treating magnesiumor magnesium alloys to preferably generate cosmetically acceptablebright textured surfaces which may be overlaid with an inorganic,transparent and/or corrosion resistant film. With reference to Table 1examples of possible compositions (or process solutions and preferredoperating conditions) for the surface treatment processing steps,subsequently to be described are set out. Table 2 illustrates possiblepre-treatment process steps (and operating conditions) for a number ofcommonly utilised alloys. Table 3 provides a useful summary of somecombinations of process steps (from Table 1) and their sequencing fortreating the surface of magnesium or magnesium alloys. TABLE 1 PROCESSSOLUTIONS Name Code Composition Temperature Phosphoric etch PE3Phosphoric acid (H₃PO₄) - 10 mol/l Ambient Nitric acid (HNO₃) - 0.5mol/l Electropolishing EP3 Phosphoric acid (H₃PO₄) - 10 mol/l 25-35° C.solution Monopropylene glycol (C₃H₆(OH)₂) - 4.5 mol/l Washing solutionPG1 Monopropylene glycol - 100% 40-60° C. Caustic washing CW25 Sodiumhydroxide (NaOH) - 6.25 mol/l   70° C. solution Passivator 1 PM1Potassium permanganate (KMnO₄) - 0.25 mol/l 15-30° C. Phosphoric acid(H₃PO₄) - to adjust pH to 5-5.5 (added as 5% H₃PO₄) Passivator 2 PC1Chromic acid (CrO₃) - 1.8 mol/l 15-30° C. Ferric nitrate(Fe(NO₃)₃.9H₂O - 0.1 mol/l Potassium fluoride (KF) 0.07-0.15 mol/lChemical blast CB1 Ferric chloride (FeCl₃) - 0.12 mol/l Ambient solutionPhosphoric acid (H₃PO₄) - 0.8 mol/l Improved chemical CB2A Ferricchloride (FeCl₃) - 0.16 mol/l Ambient blast Solution Hydrochloric acid(HCl) - 0.05 mol/l Ammonium bifluoride (NH₄HF₂) - 0.06 mol/l Ammoniumbromide (NH₄Br) - 0.17 mol/l

TABLE 2 RECOMMENDED PRE-TREATMENTS Pre-treatment Steps Forming ProcessAlloy Description Compound Conditions Die casting (for bright AZ91Degrease CW25 1-2 min or metallic finish) Rinse H₂O Phosphoric Etch PE330-60 seconds Rinse H₂O Die casting (for bright AM50/60 Degrease CW251-2 minutes or metallic finish) Rinse H₂O Phosphoric Etch PE3 30 secondsRinse H₂O Rolled sheet (for AZ31 Degrease CW25 1-2 minutes bright ormetallic Rinse H₂O finish) Phosphoric Etch PE3 Up to 2 minutes Rinse H₂OExtrusion (for bright AZ31 Degrease CW25 1-2 minutes metallic finish)Rinse H₂O All alloys (chemically As for bright metallic blasted finish)finish plus: Rinse CW25 30-45 seconds Rinse H₂O

TABLE 3 SUMMARY OF PROCESS SEQUENCE FOR ELECTROPOLISHING, METALLICFINISH AND CHEMICAL BLAST FINISH Process steps Forming Process AlloyFinish Description Compound Conditions 1. Chemical blasting (a) AZ91Chemical blast Activation CB2A 1 minute Die casting AM50 Chemicalblasting CB1 2-10 minutes Extrusion AM60 Chemical blasting 2 CB2A 1minute Rolled sheet AZ31 Caustic wash CW25 1-2 minutes Rinse H₂O (b)AZ31 Chemical blast Same as (a) above, then Extrusion Phosphoric etchPE3 1.5-2.5 minutes Rolled sheet Rinse H₂O 2. Electropolish Die castingAZ91 Bright finish No Chemical blasting EP3 8 minutes Galvanic polishEP3 15-20 seconds AC current Chemical blast Same as (a) above, then: ACcurrent EP3 2 minutes Galvanic polish EP3 3-4 minutes AC current EP315-20 seconds AM50 Bright finish No Chemical blasting EP3 6 minutes AM60Galvanic polish EP3 15-20 seconds AC current Chemical blast Same as (a)above, then: AC current EP3 1 minute Galvanic polish EP3 2-3 minutes ACcurrent EP3 15-20 seconds Rolled sheet AZ31 Bright finish No Chemicalblasting EP3 30 seconds Galvanic polish EP3 10 seconds AC current EP3 20seconds Galvanic polish EP3 10 seconds AC current Extrusion AZ31 Brightfinish No Chemical blasting EP3 50-60 seconds Galvanic polish EP3 10seconds AC current Chemical blast Same as (a) or (b) above steps then asfor bright finish EP3 50-60 seconds EP3 10 seconds Die casting AZ91Metallic finish Galvanic polish EP3 4-5 minutes Note: No AC currentC-blast metallic Same as (a) above then: Galvanic polish EP3 3-5 minutesAM50 Metallic Galvanic polish EP3 3.5-4 minutes AM60 CB metal Same as(a) above then EP3 2-3 minutes galvanic polish 3. Monopropylene glycolrinse (subsequent to steps 1, 2 or step 2 as appropriate) Die castingAZ91 Bright Rinse Extrusion AZ31 Metallic PG1 3-5 secs (one brief dip)Die casting AM50 Bright Rinse AM60 Metallic PG1 5-7 secs (with stirring)4. Caustic wash (subsequent to above steps 1-3 or 2-3 as appropriate)All All All Caustic wash CW25 20-30 seconds Rinse H₂O 5. Passivation(subsequent to above steps 1-4 or 2-4 as appropriate) Die casting AZ91Bright Passivate PC1 7-10 secs Metallic Passivate PC1 30-60 secs AM50Bright Passivate PM1 7-10 secs AM60 Metallic Passivate PC1 30-60 secsAll All All Electropassivate NH₄HF₂ 30 secs, 10-20 volts Extrusion AZ31All Passivate PC1 30-60 secs 6 Final steps (subsequent to 1-5 or 2-5 asappropriate) All All All Rinse H₂O Dry Hot air

In the tables, references to concentrations and temperatures areillustrative examples only of values which provide a preferred surfacefinish. The processes operate over a range, both of concentrations andtemperatures. Furthermore, substitutions may be made for chemicalconstituents; for instance, ethylene glycol may be used in place ofmonopropylene glycol.

Following the pre-treatment step(s), the magnesium or magnesium alloyarticle surface may be polished to a bright or metallic finish (step 6),or may be chemically “blasted” by a selective etch in step 5 to give asubstantially light, textured finish similar to shot blasting. Themethod of producing these finishes is fundamentally similar in mostcases and may be summarised in Table 3.

FIG. 1 should be read in conjunction with the foregoing tables fordetails of times, temperatures and process solutions applicable to eachstep. The examples also provide illustrative details of how variousalloys may be treated according to the present invention.

The initial method described below may have particular application tohigh aluminium content alloys (for example, alloys containing greaterthan about 3% by weight of aluminium). Although the overall conceptremains similar for low aluminium content alloys (for example, less thanabout 3% by weight), the surface finish may not be as effective as thatobtained by the low aluminium content polishing and/or brighteningmethod also described below.

High Aluminium Content Magnesium Alloys

A process of chemical or electrochemical selective etching 3accomplishes the removal, from the surface being treated, ofdiscontinuities and severe alloy component segregation. This process mayalso impart a somewhat controllable altering to the texture of thesurface. The term “etched” may be interchangeable replaced by“chemically blasted” (CB). The word “etched” may be reserved to describeprocesses as used in the pre-treatment (steps 1 to 4 in FIG. 1) phasesonly.

An electrochemical process using a modified phosphoric acid electrolytemay be able to “micro-polish” the surface to a high degree ofbrightness.

The “electropolish” step 6 illustrated in FIG. 1 and further describedin Table 3 may involve two separate treatments that may be conducted inone process vessel. The first of these may be a “galvanic polish” inwhich the article is short-circuited to a copper counter-electrodeimmersed in the EP3/EP4 (composition given in Table 1) solution. Duringthe galvanic polish there is no externally imposed current.

A second, and optional treatment may be conducted during theelectro-polishing and may comprise the imposition of an AC voltage, atabout 5 VAC. The current may not be controlled and may therefore reach avalue that depends on the surface area of the work being processed andthe characteristics of the surface and alloy. A current density of 750to 1,500 A/m² may be expected. A copper counter-electrode may be used inthe electro-chemical cell/bath.

A non-damaging method for the removal of electrolyte(s) from the surfaceafter the electro-chemical polishing step 6 may be made possible bywashing the surface with a suitable electrolyte removal component instep 7. For example, following the electrochemical polishing step 6 inwhich the EP3 electrolyte is used; a suitable wash (intermediary wash)may be a monopropylene glycol or ethylene glycol.

Step 7 may then be followed by an alkaline wash step 8 to neutralise anyacids and further remove any electrolyte from the surface. Step 8 maythen be followed by rinsing the surface with de-ionised water in step 8Aprior to the surface being treated in the passivation step 9.

The resulting treated surface may then be passivated in step 9 by theanodic deposition of a transparent corrosion resistant film upon thesurface, in which the deposition thickness may be controllable. Asuitable corrosion resistant film may, for example, be a phosphate saltfilm, and such a salt may be a preferred film as a result of its usefulcorrosion resistance properties such as water insolubility and beingslightly alkaline. The thickness of this deposited film may be varied toprovide increased corrosion resistance, as well as altering the finalsurface finish to provide a variety of surface finishes, for example, abright shiny surface (a thin layer of film), a metallic slightly dulledsurface finish (a thicker layer of film than the bright shiny finish),and a “pearl-type” surface finish (a thicker layer of film that themetallic finish). The thickness of the film is of the order of about 100nm, although it should be noted that the thickness of the film shouldpreferably not be around one quarter of the wavelength of visible lightas interference effects will occur.

Passivation of the brightened, metallic or chemically blasted surfacemay be undertaken chemically, using either of PM1 or PC1 described inTable 1 or by means of an electropassivation step. As shown in FIG. 2,in an electro-passivation step a DC voltage source of approximately10-20 volts may be applied to the article to be passivated, which ismade the anode of a circuit in a solution 14 of about 2% ammoniumbifluoride (NH₄HF₂.2H₂O) at ambient temperature, for about 30 seconds(refer to FIG. 2 for the setup). An Aluminium alloy counter-electrode 15having at least twice the surface area of the article 13 beingpassivated may be employed. Articles to be passivated are connected to aconductive busbar which is made the anode of an electrochemical cell.Such electro-passivation methods may preferably avoid the use of heavymetals and provides substantially useful corrosion resistance.

De-ionised rinse step 10 may follow the passivation step 9 in order toremove any passivation composition components, and may then be followedby a drying step 11, which may be an air drying process. It may bedesirable that a hot air dry process is employed.

Finally, this passivated surface may be treated with an inorganic,transparent and substantially corrosion resistant film, such as asolution formed by combining disodium metasilicate, a polyacrylamidecoagulant and de-ionised water (IS1—see below).

Theory of Chemical Processing Operations

Different alloys have different chemical and surface propertiesdepending on the alloy composition and its method of fabrication.Generally for castings, there can be large areas of phase segregationand sometimes several phases are present on the surface after cleaning.All cleaning, etching, polishing and post-treatment processes react withthese different phases at different rates and produce different surfacetextures that may have different residual rates of reaction in corrosiveenvironments. In the case of rolled sheet, extrusion and forgedmaterial, the metal grain size is smaller and the degree of segregationmuch lower, hence any treatment will produce a more uniform result.

The other major factors influencing the success of surface treatmentsare largely mechanical in nature. These include a variety of castingfaults, dross inclusions and surface mechanical damage. Varioustechniques have been developed to largely overcome the problems outlinedabove.

Chemical Blasting (CB)—Step 5

This is a process where iron in the process solution is reduced by themagnesium alloy and is selectively deposited on the surface as Fe orother insoluble Fe compounds. Depending on the alloy and the treatmentconditions, these deposits can be controlled to give differentdeposition sizes and patterns. When these deposits are subsequentlyremoved from the surface a series of pits and ridges remain on the metalsurface. A surface that has a more regular geometric pattern has nowreplaced the original metal surface (with random defects). Hence theappearance is enhanced.

Polishing—Step 6

This is a process that removes previously accumulated surface layers andselectively removes microscopic high points from the surface. The metalsurface becomes microscopically smoother and hence more reflective orbrighter.

There are two distinct processes included under this general heading;

Chemical Polishing—in which the component to be polished is suspended inthe polishing solution for a certain time under fixed conditions, andGalvanic Polishing—similar to the above except that the component iselectrically connected to a sheet-copper counter electrode that linesthe polishing tank and a galvanic current flows between the two to givea different polishing effect.

Sometimes the galvanic method is supplemented by the imposition of an ACcurrent at certain stages. This is done to assist in breaking diffusionstagnation and to remove surface contamination.

Alkaline Washes—Step 8

These hot strong alkaline solutions are employed in the process schemesas a convenient method for the following:

In conjunction with surfactants, to remove oil, grease and die-lube(that is, basic cleaning).

-   1. To selectively remove Al, Mn and Zn from alloy surfaces.-   2. To neutralise acidic films especially from the polishing process.-   3. To act as a pre-conditioner and activator prior to the polishing    processes.

It is recommended that two separate baths are used, one for item 1 (DGA)and the other (free of surfactant) CW25 (see Table 1) or CW10 (describedbelow) for other purposes.

MPG Wash—Step 7

This is basically a bath of pure mono-propylene glycol or ethyleneglycol run either cold or moderately hot to act as an intermediary washbetween strong phosphoric acid solutions and the alkaline wash solution.This is used so that the neutralisation reaction is less violent and mayprevent the rapid stain-producing attack of dilute phosphoric acid onmagnesium alloys. Accumulated water and acid in this solution can becontrolled by external treatment.

Passivation—Step 9

There are several methods of passivation of the finished surfaceavailable but the methods proposed by the present invention do not usechromium, are almost transparent, are easily controlled, can producemodified finishes and are effective against corrosion and handlingdamage. When used in conjunction with acrylic clear powder coatings theyprevent interface corrosion and resin degradation (yellowing). They alsoprovide a compatible, stable inter-facial layer when used in conjunctionwith inorganic sealing.

Inorganic Seal

An inorganic sealing step is used to deposit a clear Silicon (Si) basedcoating over anodised or passive bright magnesium or alloy components.The coating is applied by dipping or by spraying and by adjustment ofthe conditions different film thicknesses can be obtained. The inorganicseal is advantageously substantially transparent and gives the substrategood protection against corrosion and mechanical damage. It can be usedeffectively over only a limited number of dye coloured anodisedmagnesium substrates as there can be a colour shift due to the high pHof the inorganic seal or in some cases the dye can be destroyed.

Colouring

A limited number of colours can be applied to bright surfaces so thatthe metallic lustre is preserved. This is done by establishing a verythin (<≈100 nm), transparent film of Magnesium Oxide (MgO) on the brightsubstrate by a variety of means. It can then be dyed by a modifiedprocedure using standard Aluminium (Al) dyes.

Chemical Processing Operations

For the purpose of this discussion, non-anodised finishes are defined asdecorative finishes that are non-opaque and largely reveal the substratemetal. The appearance can be further sub-classified as bright, metallic,pearl, etc and the texture of the substrate may be classified as flat,brushed, etched etc. As well as having the normal “silver” colour of thealloy, it is possible to produce finishes that incorporate other coloursand, in general, the appearance is a “metallic” finish.

A number of procedures have been developed to produce a range offinishes and a specific procedure is chosen according to givenparameters such as;

-   1. Appearance e.g. bright.-   2. Surface texture e.g. etched.-   3. Colour e.g. silver-   4. Alloy e.g. AZ31, AM50.-   5. Alloy form e.g. cast, rolled sheet, forged.

The following set out various examples and steps involved in treatingmagnesium and its alloys to achieve a brightened surface finish. Theyare not definitive, and are examples only to illustrate preferredmethodologies.

EXAMPLE 1

A die cast plate, 140 mm×100 mm, 3 mm thick, of AM50 alloy, wasbrightened, using the following steps:

-   1. Degrease in CW25 at 70° C. for one minute-   2. Rinse in water for approximately 30 seconds-   3. Pre-etch in PE3 for 30 seconds-   4. Rinse in water-   5. Electropolish in EP3 solution in two stages—galvanic polish (no    imposed AC voltage) for six minutes, followed by an imposed AC    voltage of 5 VAC for 20 seconds.-   6. Rinse in monopropylene glycol for 5 seconds-   7. Wash in CW25 for 20 seconds at 70° C.-   8. Rinse in water-   9. Passivate, using potassium permanganate solution, PM1, for 10    seconds-   10. Rinse in water-   11. Dry using hot air

The result was a very bright finish, having a slight yellow tinge. Thedie cast structure was plainly visible.

EXAMPLE 2

A die cast plate of AZ91D alloy having the same dimensions as in example1 above, was treated using the following processes:

-   1. Degrease in CW25, 70° C., for 1 minute-   2. Rinse in water-   3. Pre-etch in PE3 for 30 seconds-   4. Rinse in water-   5. Polish in EP3 solution for 5 minutes, using a galvanic polish (no    AC voltage)-   6. Rinse in monopropylene glycol for 5 seconds-   7. Wash in CW25, 70° C. for 20 seconds-   8. Rinse in water-   9. Passivate using PC1 for 30 seconds-   10. Rinse in water-   11. Dry using hot air

The result was a metallic finish in which casting structures werevisible.

EXAMPLE 3

A die cast plate of AM50 alloy, having the same dimensions as that inexamples 1 and 2 above, was treated using the following processes:

-   1. Degrease in CW25 at 70° C. for 1 minute-   2. Rinse in water-   3. Pre-etch in PE3 for 30 seconds-   4. Wash in CW25, 70° C. for 30 seconds-   5. Rinse in water-   6. Chemical blast in three steps —1 minute in CB2A solution followed    by 7 minutes in CB1 solution and finally, an additional minute in    CB2A solution-   7. Wash in CW25, 70° C., 1 minute-   8. Polish in EP3 solution in three steps—one minute using an imposed    AC voltage of 5 VAC, 3 minutes without AC voltage, then an    additional 20 seconds using an imposed AC voltage as before.-   9. Rinse in monopropylene glycol for 5 seconds-   10. Wash in CW25, 70° C. for 20 seconds-   11. Rinse in water-   12. Passivate in PM1 for ten seconds-   13. Rinse in water-   14. Dry using hot air

The result was a bright finish, with a high degree of texturing whichobscured most of the die cast structures. Examination through amicroscope revealed a surface topography of sharp ridges adjacent torounded pits.

EXAMPLE 4

A rolled flat plate, of AZ31B alloy, 110 mm×80 mm, 1 mm thick waspre-conditioned so that it was clean of sundry corrosion and dirt. Itwas then processed as follows:

-   1. Degrease in CW25, 70° C. for one minute-   2. Rinse in water-   3. Pre-etch in PE3 for 15 seconds-   4. Rinse in water-   5. Polish using four steps—30 seconds without AC, followed by 10    seconds using an imposed AC voltage of 5 VAC, another 20 seconds    without AC and finally, a further 10 seconds with the AC voltage as    before-   6. Rinse in monopropylene glycol for 5 seconds-   7. Wash in CW25, 70° C. for 30 seconds-   8. Rinse in water-   9. Passivate in PC1 for 30 seconds-   10. Rinse in water-   11. Dry using hot air

The result was a mirror bright finish.

EXAMPLE 5

A flat plate, AM50 alloy, of the same dimensions as those in example 1above, was processed using the following treatments:

-   1. Degrease in CW25, 70° C. for one minute-   2. Rinse in water-   3. Pre-etch in PE3 for 15 seconds-   4. Rinse in water-   5. Polish in EP3 solution in two stages—galvanic polish (no imposed    AC voltage) for six minutes, followed by an imposed AC voltage of 5    VAC for 20 seconds.-   6. Rinse in monopropylene glycol for 5 seconds-   7. Wash in CW25 for 20 seconds at 70° C.-   8. Rinse in water

The plate was then passivated by an electrochemical treatment. It wasimmersed in a solution of 2% ammonium bifluoride (NH₄HF₂.2H₂O) which wascontained in a plastic tank with attached stainless steel plates on eachside. These were connected to the negative terminal of a suitable powersupply while the article itself was attached to a bus-bar that wasconnected to the positive terminal. After twenty seconds, the power wasswitched on and maintained at around 15 VDC (±about 1 VDC). Upon theapplication of the electric current the article became noticeablybrighter. There was no gas evolution. Initially the current was 20 Amps,but this decayed to around 1 Amp after about 5 seconds and finally toabout 0.3 Amps after about 30 seconds. At this time the power wasswitched off, the plate was removed from the solution, rinsed withdeionised water and dried. The article was bright and shiny inappearance with no obvious surface film. However, its passivity wasobvious when tested using reagents towards which magnesium metalarticles are normally highly reactive.

Low Aluminium Content Magnesium Alloys (Variant Process)

In addition to the above mentioned exampled and process steps, as analternative, the process may be modified to provide an optimised orsuperior treatment for low aluminium content magnesium alloys,preferably magnesium alloys containing approximately 3% aluminium (byweight) or less. In particular, it has been discovered that thisalternative surface treatment process works well with the fine-grainedAZ31 alloy in either rolled sheet or extrusion form.

A new additional process solution (EP4—described below) tailored to thespecial requirements of fine-grained AZ31 alloy in either rolled sheetor extrusion form has been developed for low electropolishing lowaluminium content magnesium alloys. The process including EP4 solutionis superior to the above described high Aluminium content method as ithas fewer process steps, superior finish, reduced cost and preferablyuses less energy intensive process conditions. When used in conjunctionwith previously defined post treatments, for example passivation, thefinished material has a more aesthetic appearance and is more corrosionresistant than previously obtainable when surface treating low Aluminiumcontent magnesium alloys using the above described high aluminiumcontent method. In addition the process can be used with greatersimplicity alongside the surface texturing or polishing process.

The following are specifications of preferred solutions and operatingconditions utilised in the low Aluminium content magnesium alloypolishing or brightening method.

Further Specifications of Solutions and Operating Conditions

Chemical Polishing solution (EP4)

750 mL MPG

115 mL H₂O

115 mL 69% HNO₃

20 mL 85% H₃PO₄

Temperature: 20-30° C.

10% Caustic Soda (Washing Solution) CW10

-   100 g NaOH

Water addition to make 1 L of the solution

Temperature: 70-80° C., optimum 75° C.

CB Solution 1 (CB1)

900 mL H₂O

50 mL 40% FeCl₃

50 mL 85% H₃PO₄

Temperature: 25-40° C.

CB1A Solution

Reduced CB1 solution

Temperature: 25-40° C.

CB2A Solution

Activator

780 mL H₂O

66 mL 40% FeCl₃

52 mL 3.5-3.7% HCl

102 mL 2% NH₄HF₂

16.7 g NH₄Br

Temperature: 20-30° C.

Mild Chemical Blasting Solution CB3

55 mL 85% H₃PO4

25 g Fe(NO₃)₃.9H₂O

11 mL 40% FeCl₃

Water addition to make 1 L of the solution

Temperature: ambient (20-25° C.)

Iron (Fe) Removal Solution FRS2

95 mL 85% HNO₃

35 g Na₂B₄O₇.5H₂O

Water addition to make 1 L of the solution

Temperature: ambient (20-25° C.)

Electropassivator BPT1

30 g Na₄P₂O₇

51.7 g Na₂B₄O₇.5H2O

7.5 NaF

Temperature: 15-30° C.

Finishes: Bright—up to 70 V (except AZ31 extrusion: up to 50 V)

(Compositions shown are for 1 litre of solution.)

Phosphoric Etch (PE3)

605 mL 85% H₃PO₄

365 mL H₂O

30 mL 69% HNO₃

Temperature: Ambient

Electropolishing Solution (EP3)

645 mL 85% H₃PO₄

323 ml monopropylene glycol: CH₃CH(OH)CH₂OH (MPG)

32 mL H₂O

Operating temperature: 35-45° C.

Important: Overheating above 50° C. is prohibited

Chemical Polishing Solution (EP4)

750 mL MPG

115 mL H₂O

115 mL 69% HNO₃

20 mL 85% H₃PO₄

Temperature: 35-45° C., optimum 40° C.

7% Caustic Soda (Washing Solution) (CW7)

70 g NaOH

Water addition to make 1 L of the solution

Temperature: 70-80° C., optimum 75° C.

Mild Chemical Blasting Solution CB3

55 mL 85% H₃PO₄

25 g Fe(NO₃)₃.9H₂O

11 mL 40% FeCl₃

Water addition to make 1 L of the solution

Temperature: ambient (20-25° C.)

Iron (Fe) Removal Solution FR1

15 mL 98% H₂SO₄

30 mL 85% H₃PO₄

Water addition to make 1 L of the solution

Temperature: ambient (20-25° C.)

Technical Grade MPG (Washing Solution) (PG1)

Operating temperature: 35-45° C.

25% Caustic Soda (Washing Solution) (CW25)

250 g NaOH

750 mL H₂O

Operating temperature: 70-80° C.

Electropassivator BPT1

30 g Na₄P₂O₇

51.7 g Na₂B₄O₇.5H₂O

7.5 NaF

Temperature: 15-30° C.

Finishes:

Bright—up to 70 V (except AZ31 extrusion: up to 50 V)

CB Solution 1 (CB1)

900 mL H₂O

50 mL 40% FeCl₃

50 mL 85% H₃PO₄

Temperature: 30-40° C.

CB1a Solution

Reduced CB1 solution

Temperature: 30-40° C.

CB2a Solution

Activator

780 mL H₂O

66 mL 40% FeCl₃

52 mL 3.5-3.7% HCl

102 mL 2% NH₄HF₂

16.7 g NH₄Br

Temperature: 20-30° C.

Inorganic Seal (IS1)

12% Na₂SiO₃ viscosity stabilised

Temperature: 55-65° C.

Preparation of 1 L of the seal:

-   a) Dissolve 1 g of “Irrigaid” polyacrylamide coagulant in 700 mL of    stirred DI water until fully dissolved-   b) Slowly add 300 mL 40% Na₂SiO₃-   c) Filter.    Chemical Polishing Solution (EP4)

750 mL MPG

115 mL H₂O

115 mL 69% HNO₃

20 mL 85% H₃PO₄

Temperature: 30-35° C.

10% Caustic Soda (Washing Solution) CW10

100 g NaOH

Water addition to make 1 L of the solution

Temperature: 70-80° C., optimum 75° C.

Mild Chemical Blasting Solution CB3

55 mL 85% H₃PO₄

25 g Fe(NO₃)₃.9H₂O

11 mL 40% FeCl₃

Water addition to make 1 L of the solution

Temperature: ambient (20-25° C.)

Iron (Fe) Removal Solution FRS1

15 mL 98% H₂SO₄

30 mL 85% H₃PO₄

Water addition to make 1 L of the solution

Temperature: ambient (20-25° C.)

Iron (Fe) Removal Solution FRS2

95 mL 85% HNO₃

35 g Na₂B₄O₇.5H₂O

Water addition to make 1 L of the solution

Temperature: ambient (20-25° C.)

If the AZ31 (or similar low aluminium content alloy) surface is notcovered in contaminants, no initial treatment to remove these isrequired. However, if the surface is covered with a protective corrosionresistant coating, then the FRS2 solution (which may for example be aniron removal solution) may be used to prepare the surface for subsequenttreatment.

Further, for this alternative process the use of mixed acids in thepolishing step may be used; compared to generally phosphoric acid onlyin the previously mentioned process for magnesium alloys having anAluminium content of approximately 3% (by weight) or greater; togetherwith a carrier solvent, such as monopropylene glycol (MPG). Thisalternative polishing step may produce an enhanced surface polish in amore controlled manner compared to the previously described process.

Phosphoric acid and predominantly nitric acid may comprise the preferredmixed acids utilised for the polishing step with low aluminium contentsurfaces. In fact, it is believed that the phosphoric acid isconsequently now used as a catalytic component, rather than as a majorconsumable component, such as the nitric acid component. The polishingstep using the EP4 solution may also be operated at approximately roomtemperature (˜20° C.); although it is appreciated that variation in theoperating temperature may increase or decrease the rate of the surfacetreatment steps. Advantageously the components of the polishing step areconsumed at lower rates, which may result in reduced frequency ofpolishing solution replenishment.

The requirement of an AC supply in the AC electrochemical polishing step6 becomes optional as the combined mixed acid and MPG provide sufficientpolishing of the surface. The intermediary wash step 7, for exampleusing MPG as previously described in the process above is also no longerrequired, and advantageously it may be that any “washing marks”resulting from the previously described intermediary process are alsoeliminated.

A variety of process steps with preferred process solutions andoperating conditions may now be provided by way of example. Steps 1 to 4may be omitted although in some cases an etching pre-treatment step maybe required to remove any external protective coating from the surface.Surface Texturing Heavy chemical blasting 1. CB2A activator 20-25 s 2.CB1A solution 3-6 min 3. FRS2 solution 2-2.5 min (with intensivestirring) 4. Water wash Mild chemical blasting 1. CB2A activator 20-25 s2. CB3 solution 3-6 min 3. FRS2 solution 2-2.5 min (with intensivestirring) 4. Water wash Surface Polishing Treatments 1. Chemicalpolishing in EP4 solution 20-90 s 2. Washing in CW10 15-45 s 3. Waterwash Post-treatment 1. Passivation (BPT1, voltage according to   desiredfinish) 2. Water wash 3. DryAn Experimental Set-Up

A brightening process was trialled and was conducted in a 2000 ml beakerat 20 to 30° C. in EP4 solution.

The bath was stirred with a mechanical stirrer and its temperaturerecorded.

At various intervals the bath conductivity was measured and smallsamples of EP4 were removed for later analysis. Weight loss measurementswere made on the Magnesium test plates at regular intervals as well asthe amount of EP4 drag-out. All measured parameters have been evaluatedas a function of the cumulative area processed.

Typical sample plate processing consisted of;

-   1. Test plates were AZ31 alloy (Spectrolite) sheets 110×80×1.2 mm.    Total area 0.0176 m² each. Plates were recycled for a number of    process runs and hence only one pre-treatment in dilute nitric acid    was given for each new plate introduced.-   2. Plate accurately weighed.-   3. Plate polished in EP4 for 2 min and drained for 10 sec.-   4. Plate washed in 10% NaOH for 15 sec.-   5. Plate rinsed in DI water for 10 to 15 sec, hot air dried and    reweighed.

The brightness of the finished plates were monitored and some plateswere passivated in BPT2 solution to obtain either bright, metallic orpearl finishes to assess this process.

Any passivated plates were stripped in dilute nitric acid afterassessment before return to the start of the process.

Results:

-   1. A total of 247 test plates were processed through the polishing    bath with no chemical replenishment during this time. The total area    processed was 4.488 m² or 2.244 m²/L.-   2. The cut-off point for the trial was defined to be when the metal    removal rate (MMR) fell to less than 60% of the initial rate.-   3. All test plates were brightened very satisfactorily up to the end    of the trial.-   4. After the trial had ended a replenishment was made to the bath    and the results measured were in close agreement with prediction.-   5. The experimental data fits the bath operating model. This model    accounts for both component consumption and dragout.-   6. Satisfactory methods have been developed for the analysis of    Magnesium, total acid, MPG, phosphate, nitrate and MRR.

This process has proven to be capable of producing excellent results onfine-grained AZ31 sheet in a very consistent manner.

The process is simple, has few steps and utilises relatively low costchemicals and simple low cost plant and equipment. The process is easilymanaged using simple laboratory tests.

The bath is easily maintained with a two-part addition mixture on aperiodic basis. General Procedure (and codes) for All Finishes 1.Pre-treatment(s) E 2. Surface texturing X 3. Surface treatment S 4.Post-treatment(s) T 5. Colouring (optional) C

EXAMPLES OF PROCEDURES

Pre-Treatments

E0-No Pre-Treatment Step E1 Step No. Operation Chemicals Temp. ° C. Timemin 1 Degrease* DGA 70-80 5 2 Water rinse* Tap 15-50 0.5-1 3 PhosphoricPE3 15-35 0.5 etch 4 Water rinse Tap 15-50 0.5-1 5 Alkaline wash DGA70-80    1-1.5 6 Water rinse Tap 15-50 0.5-1*These steps can be omitted if metal surface does not have lubricants

E2 Step No. Operation Chemicals Temp. ° C. Time min 1 Degrease CW2570-80 2-3 2 Water rinse Tap 15-50 0.5-1  Illustrative Surface Texturing (CB)

X0-No Surface Texturing Step X1 Step No. Operation Chemicals Temp. ° C.Time min 1 Activation CB2a 30-40 0.3 2 Chemical blast CB1 (CB1a) 30-401-5 3 Alkaline wash CW25 70-80 5 4 Water rinse Tap 15-50 0.5-1  

X2 Step No. Operation Chemicals Temp. ° C. Time min 1 Chemical Blast CB1(CB1a) 30-40 1-6 2 Alkaline wash CW25 70-80 5 3 Water rinse Tap 15-500.5-1  

X3 Step No. Operation Chemicals Temp. ° C. Time min 1 Activation CB2a30-40 0.3-0.5 2 Chemical blast CB3 30-40   1-5   3 Iron removal FR120-40 0.5-1.5 4 Water rinse Tap 15-50 0.5-1  

Illustrative Surface Treatments S1 Step No. Operation Chemicals Temp. °C. Time min 1 Chemical polish EP3 35-45 2 2 Galvanic polish EP3 35-452-3 3 Chemical polish EP3 35-45   1-1.5 4 Glycol wash PG1 35-40 0.3 5Alkaline wash CW25 70-80 1.5-2   6 Water wash Tap 20-40 0.5-1  

S2 Step No. Operation Chemicals Temp. ° C. Time min 1 AC polish EP335-45 1 2 Galvanic polish EP3 35-45 3-4 3 AC polish EP3 35-45 0.3-0.5 4Glycol wash PG1 35-40 0.3 5 Alkaline wash CW25 70-80 0.5-1 6 Water washTap 20-40 0.5-1

S3 Step No. Operation Chemicals Temp. ° C. Time min 1 Chemical polishEP3 35-45 2 2 Galvanic polish EP3 35-45 1-1.5 3 AC polish EP3 35-450.2-0.5 4 Glycol wash PG1 35-40 0.25-0.3 5 Alkaline wash CW25 70-800.5-1 6 Water wash Tap 20-40 0.5-1

S4 Step No. Operation Chemicals Temp. ° C. Time min 1 Chemical polishEP4 35-45 0.3-2 2 Alkaline wash CW7 70-80 0.2-0.75 3 Water wash Tap20-40 0.5-1

Illustrative Post-treatments (Passivation) T1 (Bright) Time Step No.Operation Chemicals Temp. ° C. Voltage V min 1 Electro- BPT1 15-30 40-702 passivation 2 Water rinse DI water 15-30 — 0.5-1 3 Hot air dry —70-110 1

T2 (Metallic) Time Step No. Operation Chemicals Temp. ° C. Voltage V min1 Electro- BPT1 15-30 90-100 2 passivation 2 Water rinse DI water 15-30— 0.5-1 3 Hot air dry — 70-110 — 1

T3 (Pearl) Time Step No. Operation Chemicals Temp. ° C. Voltage V min 1Electro- BPT1 1514 30 115-125 2 passivation 2 Water rinse DI water 15-30— 0.5-1 3 Hot air dry — 70-110 — 1Treatment Component Table

Procedure codes are shown in table below. Finish/ AZ31 extrusion Alloyor sheet* AM50 cast AZ91 cast Flat Bright E0 + S4 + T1 E1 + S2 + T1 E1 +S1 + T1 Metallic E0 + S4 + T2 E1 + S2 + T2 E1 + S1 + T2 Pearl E0 + S4 +T3 E1 + S2 + T3 E1 + S1 + T3 CB Bright E0 + X3 + S4 + T1 E1 + X1 + S2 +T1 E1 + X1 + S1 + T1 Metallic E0 + X3 + S4 + T2 E1 + X1 + S2 + T2 E1 +X1 + S1 + T2 Pearl E0 + X3 + S4 + T3 E1 + X1 + S2 + T3 E1 + X1 + S1 + T3Results from Salt Spray (Corrosion testing) of Treated Surfaces:

Salt spray testing was conducted on samples of AZ31 alloy sheet orextrusion which had been brightened or polished according to the abovedescribed low Aluminium content method to determine resistance tocorrosion.

Sample Preparation: The samples were prepared using followingtechnique: 1. No pre-treatment for clean metal (sheet or extrusion) 2.Surface cleaning by FRS2 solution (Spectrolite sheet) 3. CB2A* 20 s 4.CB1A (heavy blasting) or CB3 (mild blasting)* 3 min 5. FRS2* 2 min 6.EP4 1 min 7. CW10 15-30 s 8. Water wash 9. BPT1 1 min 10. Water wash 11.Drying 12. Acrylic powder coating#*For CB finishes only#When required

List of Samples Passiva- No of ting Sample Ahoy samples Finish voltage,V Commentary CN1 AZ31, 2 Mild CB, 50 Powder Spectrolite bright, coatedsheet CB2A + CB3 CN2 AZ31 2 Coarse CB, 50 Powder bright, coated CB2A +CB1A CN3 AZ31 2 Mild CB, 50 Powder bright, coated CB2A + CB3 CN4 AZ31 2Mild CB, 90 Powder metallic, coated CB2A + CB3 coated CN5 AZ31 2 MildCB, pearl, 120  Powder CB2A + CB3 coated CN6 AZ31 2 Flat, bright 50Powder coated CN7 AZ31, 2 Mild CB, 50 Powder extrusion bright, coatedCB2A + CB3 CN8 AZ31, 2 Coarse CB, 50 Powder extrusion bright, coatedCB2A + CB1A CN9 AZ31, 2 Flat, bright 50 Powder extrusion coated CN10AZ31, 2 Heavy CB, 50 Powder Spectrolite bright, coated sheet CB2A + CB1ACN11 AZ31 2 CB, bright 50 No powder coating CN12 AZ31 2 CB, metallic 90No powder coating CN13 AZ31 2 CB, pearl 120  No powder coating

Experimental Results Hours in Protective salt Sample Alloy FinishPassivation coating spray Commentary Passivated metal (passivated, noprotective acrylic coating) CN11 AZ31 CB, BPT1 No coating 24 Significantpitting bright corrosion on front side. Rear side: no corrosion CN12AZ31 CB, BPT1 No coating 24 As above metallic CN13 AZ31 CB, pearl BPT1No coating 24 As above Bright finish (passivated, acrylic coating) CN1AZ31, Mild CB, BPT1 Powder coating 48 Heavy corrosion Spectrolitebright, on the top and sheet CB2A + on the bottom CB3 of the sample.Uncoating of acrylic layer CN2 AZ31 Coarse BPT1 Powder coating 48 Asabove CB, bright, CB2A + CB1A CN3 AZ31 Mild CB, BPT1 Powder coating 48As above bright, CB2A + CB3 CN6 AZ31 Flat, BPT1 Powder coating 48 Heavycorrosion bright CN7 AZ31, Mild CB, BPT1 Powder coating 24 Pittingcorrosion extrusion bright, The sample could CB2A + endure additionalCB3 24 hours. Corrosion was caused by uneven powder coating CN8 AZ31,Coarse BPT1 Powder coating 48 Heavy filiform extrusion CB, corrosionbright, Sample had CB2A + imperfect powder CB1a coating: brown colour onthe top of CB hills CN9 AZ31, Flat, BPT1 Powder coating 48 Heavy bottomextrusion bright corrosion. Uncoating of acrylic layer CN10 AZ31, HeavyBPT1 Powder coating 48 As above Spectrolite CB, sheet bright, CB2A +CB1A Metallic finish (passivated, acrylic coating) CN4 AZ31 Mild CB,BPT1 Powder coating 168  Corroded from metallic, edges with CB2A +uncoating of CB3 acrylic layer Passivated samples + acrylic powdercoating (local) CN5 AZ31 Mild CB, BPT1 Powder coating 336  Middle partof pearl, samples in good CB2A + conditions. Edge CB3 corrosion onlyCorrosion Testing Conclusions

Corrosion resistance of passivated (BPT1) and powder coated (optionally)AZ31 samples was tested. Spectrolite sheet and extrusion were used inthe experiments. Tolerance to salt spray 1. Sheet (bright, notpassivated, powder coated) less than 24 hrs (from previous experiments)2. Sheet (passivated, not powder coated) up to 24 hours 3. Sheet orextrusion (bright, passivated, powder  48 hrs coated) 4. Sheet(metallic, passivated, powder coated) 168 hrs 5. Sheet (pearl,passivated, powder coated) 336 hrs

Relatively low corrosion resistance of bright passivated AZ31 alloypresumably is due to uneven powder coating (extrusion) and smallthickness of plates (0.8 mm sheet). The latter produced centres ofcorrosion on the sample edges.

Aspects of the present invention have been described by way of exampleonly and it should be appreciated that modifications and additions maybe made thereto without departing from the scope thereof as defined inthe appended claims.

1. A method of polishing and/or brightening a magnesium or magnesiumalloy surface composing the steps of: i) polishing the surface, and ii)passlvating the polished surface, wherein the polishing step is carriedout by a chemical polish and/or electro-chemical polish while saidsurface is immersed in a polishing composition of one or more of thefollowing components; a phosphoric acid solution, monopmpylene glycol,ethylene glycol, dnd nitric acid.
 2. A method of polishing and/orbrightening a magnesium or magnesium alloy surface s claimed in claim 1,wherein the method further comprises an initial step of pre-treatingsaid surface to remove surface contaminants.
 3. A method of polishingand/or brightening a magnesium or magnesium alloy surface s claimed inclaim 2, wherein said pre-treating step comprises chemically etchingsaid surface and/or degreasing said surface.
 4. A method of polishingand/or brightening a magnesium or magnesium alloy surface claimed inclaim 2, wherein surface contaminants are removed during thepre-treatment step by contacting said surface with one or moredegreasing component, such as sodium hydroxide.
 5. A method of polishingand/or brightening a magnesium or magnesium alloy surface as claimed inclaim 3, wherein said chemical etching component is nitric acid solutionand/or phosphoric acid.
 6. A method of polishing and/or brightening amagnesium or magnesium alloy surface as claimed in claim 1, wherein saidchemical polish and/or electrochemical polish removes surface layersand/or reduces microscopic high points from the surface.
 7. A method ofpolishing and/or brightening a magnesium or magnesium alloy surface asclaimed in claim 1, wherein said electro-chemical polish is a galvanicelectrolysis.
 8. A method of polishing and/or brightening a magnesium ormagnesium alloy surface as claimed in claim 1, wherein saidelectrochemical process further includes the supply of an externalvoltage to said surface.
 9. A method of polishing and/or brightening amagnesium or magnesium alloy surface as claimed in claim 1, whereinduring said electro-chemical polish an electrolyte anti-stagnation meansis utilised or an AC voltage is applied to the electrolyte containingsaid surface.
 10. A method of polishing and/or brightening a magnesiumor magnesium alloy surface as claimed in claim 9, wherein saidelectrolyte anti means is an electrolyte stirrer and/or an ultrasonicwave generating means.
 11. A method of polishing and/or brightening amagnesium or magnesium alloy surface as claimed in claim 1, wherein saidpolishing step is followed by an intermediary wash removing at leastsome of the chemical and/or electrolyte solution from said surface. 12.A method of polishing and/or brightening a magnesium or magnesium alloysurface as claimed in claim 11, wherein said intermediary wash iscarried out in a composition containing monopropylene glycol and/orethylene glycol.
 13. A method of polishing and/or brightening amagnesium or magnesium alloy surface as claimed in claim 1, wherein saidpolishing step is followed by an alkaline wash.
 14. A method ofpolishing and/or brightening a magnesium or magnesium alloy surface asclaimed in claim 11, wherein said intermediary wash is followed by analkaline wash.
 15. A method of polishing and/or brightening a magnesiumor magnesium alloy surface as claimed in claim 13, wherein said alkalinewash substantially neutralises acids and/or substantially removesAluminium, Manganese or Zinc from said surface.
 16. A method ofpolishing and/Or brightening a magnesium or magnesium alloy surface asclaimed in claim 13, wherein said alkaline wash is carried out in acomposition containing sodium hydroxide
 17. A method of polishing and/orbrightening a magnesium or magnesium alloy surface as claimed in claim1, wherein said passivating step provides a substantially corrosionresistant and/or water insoluble surface coating or film.
 18. A methodof polishing and/or brightening a magnesium or magnesium alloy surfaceas claimed in claim 17 wherein said substantially corrosion resistantand/or water insoluble surface coating or film is a phosphate saltcoating or film.
 19. A method of polishing and/or brightening amagnesium or magnesium alloy surface as claimed in claim 17, whereinsaid passivating step voltage is varied to alter said substantiallycorrosion resistant and/or water insoluble surface coating or filmthickness.
 20. A method of polishing and/or brightening a magnesium ormagnesium alloy surface as claimed in claim 1, wherein an inorganicmaterial coating or sealer is applied to said substantially corrosionresistant and/or water insoluble surface coating or film.
 21. A methodof polishing and/or brightening a magnesium or magnesium alloy surfaceas claimed in claim 18, wherein said inorganic material coating orsealer is substantially transparent and/or substantially providescorrosion protection and/or at least provides some protection frommechanically induced damage.
 22. A method of polishing and/orbrightening a magnesium or magnesium alloy surface as claimed in claim20, wherein said inorganic material coating or sealer is a silicon basedcomposition, such as a disodium metasilicate, and a polyacrylamidecoagulant in de-ionised water.
 23. A method of polishing and/orbrightening, a magnesium or magnesium alloy surface as claimed in claim1, wherein said passivating step and/or said inorganic material coatingor sealer step is followed by a surface drying step.
 24. A method ofpolishing and/or brightening a magnesium or magnesium alloy surface asclaimed in claim 1 including the pre-treatment steps of: a. immersingthe surface in an iron based solution, b. activating said surface withsaid Iron based solution, wherein said iron based solution is reduced tothereby deposit iron on said surface, c. etching said surface with anetch composition to modify the activated surface layer, d. strippingiron deposits from said surface with an iron removal composition, and e.washing said surface to substantially remove compositions remaining onsaid surface.
 25. A method of polishing and/or brightening a magnesiumor magnesium alloy surface as claimed in claim 24, wherein saidactivator is a solution selected from the following; ferric chloride,hydrochloric acid, ammonium bifluoride and ammonium bromide.
 26. Amethod of polishing and/or brightening a magnesium or magnesium alloysurface as claimed in claim 24, wherein said etch composition isselected from the following; ferric chloride; ferric chloride andphosphoric acid solution, or a reduce solution of ferric chloride andphosphoric acid.
 27. A method of polishing and/or brightening amagnesium or magnesium alloy surface as claimed in claim 24, whereinsaid iron removal composition is selected from the following; nitricacid and sodium borate in solution or nitric acid and phosphoric acid insolution.
 28. A method of polishing and/or brightening a magnesium ormagnesium alloy surface as claimed in claim 24, wherein said step ofwashing said surface is carried out with a water wash or an alkalinewash.
 29. A method of polishing and/or brightening a magnesium ormagnesium alloy surface as hereinbefore described and with reference toany one of the accompanying drawings.
 30. A magnesium or magnesium alloysurface polished or brightened according to the method substantially ashereinbefore described and with ret to any one of the accompanyingdrawings.